1. Field of the Invention
The present invention relates to a safety switch and, more particularly, to such a safety switch, which uses a S-shaped metal spring plate to guide and support a push and pull bar, enabling the On/Off control bimetal contact plate to automatically trip off upon an overload.
2. Description of the Related Art
FIG. 7 shows a conventional seesaw switch, which comprises a switching handle 11 (which has a lamp bulb installed therein), and a press rod 110 supported on a spring in a bottom hole of the switching handle 11 and moved with the switching handle 11 to move a movable metal contact plate 12 between the xe2x80x9cOnxe2x80x9d position and the xe2x80x9cOffxe2x80x9d position. This structure of seesaw switch is a passive switch that cannot automatically trip off upon an overload. When an overload occurred, the circuit tends to be burned out. In order to eliminate this problem, switches with protective circuit means, i.e., safety switches are developed. FIGS. 8A and 8B show a safety switch according to the prior art. This structure of safety switch comprises a housing 10, a switching handle 13 pivoted to the housing 10, three metal contact plates, namely, the first metal contact plate 15, the second metal contact plate 16, and the third metal contact plate 17 respectively installed in the bottom side of the housing 10, a bimetal contact plate 170, the bimetal contact plate 170 having a fixed end fixedly fastened to the third metal contact plate 17 and a free end provided with a contact 171 adapted for contacting a contact 160 at the second metal contact plate 16 to close the circuit, a link 14 coupled between one end of the switching handle 13 and the free end of the bimetal contact plate 170 and driven by the switching handle 13, to move the contact 171 of the bimetal contact plate 170 toward or away from the contact 160 of the second metal contact plate 16, and a curved spring plate 18 fixedly connected between one peripheral sidewall of the housing 10 and the free end of the bimetal contact plate 170 and adapted for controlling the moving distance of the free end of the bimetal contact plate 170. When the temperature of the bimetal contact plate 170 surpasses a predetermined high value due to an overload, it is caused to deform and to trip off from the contact 160 of the second metal contact plate 16. This structure of safety switch is still not satisfactory in function because of the following drawbacks:
1. When the switching handle 13 jammed or stopped by an external pressure, or the curved spring plate 18 was worn-out, the bimetal contact plate 170 cannot trip off upon an overload.
2. The bimetal contact plate 170 automatically trips off when the temperature surpassed a trip-off temperature (normally 100xc2x0xcx9c150xc2x0 C.). When turning the switching handle 13 to the xe2x80x9cOnxe2x80x9d position at this time, the bimetal contact plate 170 will automatically trip off if the power of deformation of the bimetal contact plate 170 surpasses the spring power of the curved spring plate 18. If the user holds the switching handle 13 in the xe2x80x9cOnxe2x80x9d position with force to keep the contact 171 of the bimetal contact plate 170 in contact with the contact 160 of the second metal contact plate 16 at this time, the circuit may be caused to burn out.
3. Because the bimetal contact plate 170 is a planar structure for one-way trip-off functioning only, the curved spring plate 18 must be used to achieve bi-directional trip-off functioning. However, it is difficult to control the spring power of the curved spring plate 18 during its fabrication. If the spring power of the curved spring plate 18 does not match, the bimetal contact plate 170 will be unable to function normally.
FIGS. 9A and 9B show a safety switch with overcurrent protection. This structure of safety switch comprises a switching handle 20, an electrically insulative driving element 21 and a link 22 coaxially pivoted to one end of the switching handle 20, a spring member 24 connected between a fixed point and the other end of the driving element 21, and a bimetal contact plate 23. The bimetal contact plate 23 has a fixed end fixedly fastened to a first metal contact plate connected to one terminal of power supply, and a free end connected to the other end of the link 22 and moved with the link 22 relative to a contact at a second metal contact plate connected to the other terminal of power supply. When an overcurrent occurred, the bimetal contact plate 23 is heated to deform and to trip off from the contact of the second metal contact plate, and at the same time the driving element 21 is moved upwards with the free end of the bimetal contact plate 23 and forced into engagement with a curved positioning portion 25 in the housing of the safety switch to hold the bimetal contact plate 23 in the xe2x80x9cOffxe2x80x9d position. This structure of safety switch still has drawbacks. If the switching handle 20, the driving element 21, or the link 21 fails to function normally, the bimetal contact plate 23 will be unable to trip off upon an overload. Further, the spring power of the spring member 24 must be accurately controlled during its fabrication. If the spring power of the spring member 24 does not match, the driving element 21 will be unable to function properly.
Therefore, it is desirable to provide a safety switch that eliminates the aforesaid drawbacks.
The present invention has been accomplished to provide a safety switch, which eliminates the aforesaid drawbacks. It is one object of the present invention to provide a safety switch, which automatically trips off upon an overload. It is another object of the present invention to provide a safety switch, which has a simple structure, and achieves the auto trip-off functioning of a conventional complicated fuseless mechanical switch. It is still another object of the present invention to provide a safety switch, which can be smoothly accurately switched between the xe2x80x9cOnxe2x80x9d position and the xe2x80x9cOffxe2x80x9d position. To achieve these and other objects of the present invention, the safety switch comprises a housing holding two metal contact plates, which are respectively connected to the two opposite terminals of power supply, a bimetal contact plate suspended from one metal contact plate, a switching handle pivoted to the housing and adapted for moving the bimetal contact plate between the xe2x80x9cOnxe2x80x9d position to close the circuit between the two metal contact plates and the xe2x80x9cOffxe2x80x9d position to open the circuit between the two metal contact plates, a push and pull bar pivoted to one end of the switching handle and driven by the switching handle to move the bimetal contact plate between the xe2x80x9cOnxe2x80x9d position and the xe2x80x9cOffxe2x80x9d position, and a S-shaped metal spring plate adapted for supporting and guiding movement of the push and pull bar for enabling the bimetal contact plate to trip off automatically upon an overload despite of the positioning of the push and pull bar or the switching handle.